The invention relates to a security element comprising at least one display element for visual display of information which cannot be reset.
It is known to realize display devices with the aid of colored liquids which may be displaced between positions which are not visible and visible for a viewer. An efficient means for displacing liquids is the so-called electrowetting wherein voltage is supplied to an electrode which is immediately adjacent to a liquid droplet, so that surface energy and therefore surface tension of the liquid is increased. Depending on the configuration, this may cause that the liquid spreads over the electrode or even covers it completely, depending on the geometry and nature of surface of the electrode and relative position of liquid and electrode also with respect to the earth gravity field. It is known to provide the electrodes with a hydrophobic coating in order to achieve a maximum effect in this way between the state of the liquid when the electrode is not charged and the electrode is charged. It is possible already with these aforementioned means to realize a droplet movement without reverting to further functional elements.
In case of a single droplet, variation of the surface energy causes a variation of the contact angle at which the droplet is wetting its support. The interrelation between field strengths and variation of contact angle is described by the Lipmann-Young equation and is known to the skilled person.
Thus, by a local variation of the electric field strength, the local surface energy of the liquid and therefore the geometry of a droplet can be varied locally. In particular, a movement of the droplet can occur by creation of a locally increased surface energy, so that it preferably extends into the corresponding region characterized by the presence of an electric field. Contrarily, liquid is pushed out of a region in which it is not influenced and therefore comprises a lower surface energy since it tends to the geometry of lowest energy, namely that of a sphere, in this region. A net transport of liquid from the region of lower into the region of higher field strength results thereby.
A plurality of different display devices is already realized on this basis. Typically, it was always the objective to produce an electronically controllable bistabile display element which selectively assumes one state or the other. Thus, it is for example known to reciprocate a liquid between a first and a second volume which are connected via a duct with the aid of the effect of electrowetting, wherein each volume comprises an associated electrode so that the transport direction results respectively from the ratio of field strengths between both electrodes. It is preferred to supply potential to only that electrode in which direction the liquid is to be conveyed.
For realizing a security element, it is desirable, for example, to uncover or to prevent an undesired manipulation, that a change of state which has once occurred cannot be reversed.
If this would be possible on basis of the electrowetting technology described in the introductory, the whole wealth of experience documented in the prior art could be used to realize security elements which for example display exceeding of storage temperatures by means of a color change by displacing a colored fluid in a very simple and intuitive manner. Thus, the often technically complex devices which had been used for the aforementioned purposes until now, could be renounced which furthermore often are not able to guard against manipulation without permission.
A display device which cannot be reset is for example known from U.S. Pat. No. 7,310,080 B2, wherein non-resettability is achieved by the fact that a lens cannot be displaced from a position once assumed in which for example a text message can be read, back to a starting position. It can further be taken from the reference that fixation of the lens can be made by means of any locking devices, without specifying them in detail.
US 2009/0042065 A1 discloses a security element wherein a porous barrier fluidically separates a first and a second volume. An electrolyte is provided in said first volume which, due to a security relevant event, passes the barrier and contacts electrodes in said second volume. Due to a chemical reaction between electrolyte and electrode, an electric voltage drop occurs at the electrodes that serves as an indicator of the security relevant event.
DE 10 2008 020 133 A1 discloses a security display element wherein two volumes are connected via a duct, wherein a unified electrode covers both volumes as well as the connecting duct.